As worldwide electric power demands continue to increase significantly, utilities have struggled to meet these increasing demands both from a power generation standpoint as well as from a power delivery standpoint. Delivery of power to users via transmission and distribution networks remains a significant challenge to utilities due to the limited capacity of the existing installed transmission and distribution infrastructure, as well as the limited space available to add additional conventional transmission and distribution lines and cables. This is particularly pertinent in congested urban and metropolitan areas, where there is very limited existing space available to expand capacity.
Flexible, long-length power cables using high temperature superconductor (HTS) wire are being developed to increase the power capacity in utility power transmission and distribution networks, while maintaining a relatively small footprint for easier installation and using environmentally clean liquid nitrogen for cooling. For this disclosure, HTS is defined as a superconductor with a critical temperature at or above 30 K, which includes materials such as yttrium- or rare-earth-barium-copper-oxide (i.e., YBCO); thallium-barium-calcium-copper-oxide; bismuth-strontium-calcium-copper-oxide (henceforth referred to as BSCCO); mercury-barium-calcium-copper-oxide; and magnesium diboride (MgB2). YBCO has a critical temperature approximately 90 K. BSCCO has a critical temperature of approximately 90 K in one composition and approximately 110 K in a second composition. MgB2 has a critical temperature of up to approximately 40 K. These composition families are understood to include possible substitutions, additions and impurities, as long as these substitutions, additions and impurities do not reduce the critical temperature below 30° K. Such HTS cables allow for increased amounts of power to be economically and reliably provided within congested areas of a utility power network, thus relieving congestion and allowing utilities to address their problems of transmission and distribution capacity.
An HTS power cable uses HTS wire as the primary conductor of the cable (i.e., instead of traditional copper conductors) for the transmission and distribution of electricity. The design of HTS cables results in significantly lower series impedance, when compared to conventional overhead lines and underground cables. Here the series impedance of a cable or line refers to the combination of resistive impedance of the conductors carrying the power, and the reactive impedance associated with the cable architecture or overhead line. For the same cross-sectional area of the cable, HTS wire enables a three to five times increase in current-carrying capacity when compared to conventional alternating current (AC) cables; and up to a ten times increase in current-carrying capacity when compared to conventional direct current (DC) cables.
An HTS transformer may use HTS wire as the primary conductor in the transformer (i.e., instead of traditional copper conductors) for the transformation of ac power from one voltage and current level to another. See Current Limitation in High Temperature Superconducting Transformers and Impact on the Grid, E. Serres et al., CIGRE Session 2000, 12-205. HTS transformers can be designed with significantly lower impedance in their superconducting operating state, when compared to conventional transformers. In addition to their reduced impedance, HTS transformers enable many benefits, including lower ac losses, significantly reduced footprint, elimination of oil in favor of non-flammable and environmentally non-contaminating liquid nitrogen.
In addition to capacity problems, another significant problem for utilities resulting from increasing power demand (and hence increased levels of power being generated and transferred through the transmission and distribution networks) are increased “fault currents” resulting from “faults”. Faults may result from network device failures, acts of nature (e.g. lightning), acts of man (e.g. an auto accident breaking a power pole), or any other network problem causing a short circuit to ground or from one phase of the utility network to another phase. In general, such a fault appears as an extremely large load materializing instantly on the utility network. In response to the appearance of this load, the network attempts to deliver a large amount of current to the load (i.e., the fault). Any given link in the network of a power grid may be characterized by a maximum fault current that may flow, in the absence of fault current limiting measures, during the short circuit that precipitates the maximum fault condition. The fault currents may be so large in large power grids that without fault current limiting measures, most electrical equipment in the grid may be damaged or destroyed. The conventional way of protecting against fault currents is to rapidly open circuit breakers and completely stop the current and power flow.
Detector circuits associated with circuit breakers monitor the network to detect the presence of a fault (or over-current) situation. Within a few milliseconds of detection, activation signals from the detector circuits may initiate the opening of circuit breakers to prevent destruction of various network components. Currently, the maximum capability of existing circuit breaker devices is approximately 80,000 amps, and these are for transmission level voltages only. Many sections of the utility network built over the previous century were built with network devices capable of withstanding only 40,000-63,000 amps of fault current. Unfortunately, with increased levels of power generation and transmission on utility networks, fault current levels are increasing to the point where they will exceed the capabilities of presently installed or state-of-the-art circuit breaker devices (i.e. be greater than 80,000 amps) both at distribution and transmission level voltages. Even at lower fault current levels, the costs of upgrading circuit breakers from a lower level to a higher level across an entire grid can be very high. Accordingly, utilities are looking for new solutions to deal with the increasing level of fault currents. In most cases, it is desirable to reduce fault currents by at least 10% to make a meaningful improvement in the operation of a grid. One such solution in development is a device called an HTS fault current limiter (FCL).
An HTS FCL is a dedicated device interconnected to a utility network that reduces the amplitude of the fault currents to levels that conventional, readily available or already installed circuit breakers may handle. See High-Temperature Superconductor Fault Current Limiters by Noe and M. Steurer, Supercond. Sci. Technol. 20 (2007) R15-R29. Such HTS FCLs have typically been configured out of short rigid modules made of solid bars or cylinders of HTS material that have very high resistance when they are driven over their superconducting critical current into a resistive state. Unfortunately, such standalone HTS FCLs are currently quite large and expensive. Space is particularly at a premium in substations in dense urban environments where HTS cables are most needed. Utilities may also use large inductors, but they may cause extra losses, voltage regulation and grid stability problems. And, unfortunately, pyrotechnic current limiters (e.g., fuses) need replacement after every fault event. Further, while new power electronic FCLs are under development, there are questions about whether they can be fail-safe and whether they can be extended reliably to transmission voltage levels.
To allow HTS cables to survive the flow of fault currents, a significant amount of copper may be introduced in conjunction with the HTS wire, but this adds to the weight and size of the cable. See Development and Demonstration of a Long Length HTS Cable to Operate in the Long Island Power Authority Transmission Grid by J. F. Maguire, F. Schmidt, S. Bratt, T. E. Welsh, J. Yuan, A. Allais, and F. Hamber, to be published in IEEE Transaction on Applied Superconductivity.
Often, copper fills the central former in the core of the HTS cable around which the HTS wire is helically wound, and this prevents the core from being used as a passage for the flow of liquid nitrogen. Alternatively and especially for multi-phase cables, copper wires may be mixed in with the HTS wires within the helically wound layers of the cable. These copper wires or structures may be electrically in parallel with the HTS wires and may be called “copper shunts” within the HTS cable. In the presence of a large fault current that exceeds the critical current of the HTS wires of the cable, they quench or switch to a resistive state that can heat from resistive I2R losses (where I is the current and R is the resistance of the cable). The “copper shunts” may be designed to absorb and carry the fault current to prevent the HTS wires from over-heating. The amount of copper utilized may be large enough so that the total resistance of the copper in the cable is comparatively small and has a negligible effect in reducing the level of the fault current. Copper may be defined to mean pure copper or copper with a small amount of impurities such that its resistivity is comparatively low in the 77-90 K temperature range (e.g., <0.5 microOhm-cm, or as low as 0.2 microOhm-cm.
In the European SUPERPOLI program (See SUPERPOLI Fault-Current Limiters Based on YBCO-Coated Stainless Steel Tapes by A. Usoskin et al., IEEE Trans. on Applied Superconductivity, Vol. 13, No. 2, June 2003, pp. 1972-5; Design Performance of a Superconducting Power Link by Paasi et al., IEEE Trans. on Applied Superconductivity, Vol. 11, No. 1, March 2001, pp. 1928-31; HTS Materials of AC Current Transport and Fault Current Limitation by Verhaege et al., IEEE Trans. on Applied Superconductivity, Vol. 11, No. 1, March 2001, pp. 2503-6; and U.S. Pat. No. 5,859,386, entitled “Superconductive Electrical Transmission Line”), superconducting power links were investigated that may also limit current.
Following the typical approach for earlier standalone FCLs, this program investigated rigid solid rods or cylinders of HTS material that formed modules or busbars for the power link. A typical length of a module or busbar was 50 cm to 2 meters. In a second approach, coated conductor wire was used in which YBCO material was coated on high resistance stainless steel substrates. A gold stabilizer layer was used, but it was kept very thin to keep the resistance per length as high as possible. The wire was helically wound on a rigid cylindrical core that formed another option for a module or busbar for the power link. In response to a fault current, both these modules switch to a very highly resistive state to limit the current. The concept proposed in the SUPERPOLI program to create a longer length cable was to interconnect the rigid modules with flexible braided copper interconnections. See U.S. Pat. No. 5,859,386, entitled “Superconductive Electrical Transmission Line”. The possibility of designing and fabricating a long-length continuously flexible cable with fault-current-limiting functionality using lower resistance and higher heat capacity wires, and hence a lower level of local heating, was not considered. Nor was the possibility of additional grid elements that could optimize the functionality of the link.
It is desirable to improve the manner in which an electric grid can handle fault currents and to provide an improved alternative to the use of standalone FCLs or other fault current limiting devices e.g., high resistance-per-length fault-current limiting busbars. HTS transformers that incorporate a current limiting functionality offer a cost-effective way to do this by avoiding the necessity for separate and costly fault-current-limiting devices in crowded utility substations. Such current limiting transformers may have improved functionality if they are configured in a grid in an optimal way to be described in this disclosure. A practical long-length flexible HTS power cable that incorporates fault current limiting functionality would also provide major benefits in establishing high capacity, low footprint and environmentally clean power transmission and distribution, while at the same time avoiding the necessity for separate and costly fault-current-limiting devices in crowded utility substations.